Optical interconnections offer several significant advantages over their electrical counterparts, such as large signal bandwidths and reduced propagation delay. When circuit arrays are formed at the wafer scale level of integration, the advantages of optical interconnections over their electrical counterparts become even more pronounced.
Low loss multilayer integrated optical waveguides using optically transparent polyimide as an embedding material and as a waveguide dielectric have been proposed in the prior art. Standard lithography processes, together with dry etching, have been used to create experimental integrated optical waveguides.
By way of example, optical waveguiding in poly(methyl methacrylate), polycarbonate and polyimide has been reported by Franke and Crowe in an article entitled "Optical waveguiding in Polyimide" SPE Vol. 651 Integrated Optical Circuit Engineering III, 1986, pp. 102-107. Amongst the polyimides studied was a polyamide-imide. Styrene and benzoin were found to be soluble in solutions of this resin and the carefully dried mixture could be first uv lithographed and then thermally cured to develop and fix a pattern on substrates such as glass or fused silica.
Kokubun, Baba and Iga reported their work relating to a "Silicon Optical Printed Circuit Board for Three-Dimensional Integrated Optics" in Electronics Letters, Vol. 21, No. 11, (1985), pp. 508-9. These workers cured a monomer blend (styrene and benzyl methacrylate) in Vee grooves in a silicon substrate.
Sullivan and Husain in a paper entitled "Guided-wave Optical Interconnects for VLSI Systems", SPIE Vol. 881 Optical Computing and nonlinear Materials, 1988, pp. 172-176 describe the use of negative-acting photosensitive polyimide to improve the smoothness of the sidewalls in low loss optical interconnects.
Sullivan reports further in an article entitled "Optical Waveguide Circuits for Printed Wire Board Interconnections", SPIE Vol. 994 Optoelectronic Materials, Devices, Packaging and Interconnects II, 1988, pp. 92-100. This article describes optical waveguides formed of optical quality polyimide covered with a silicon dioxide cladding.
Christiansen, in an article entitled "Plasma-etched Polymer Waveguides for Intrachip Optical Interconnects", SPIE Vol. 836 Optoelectronic Materials, Devices, Packaging and Interconnects, 1987, pp. 359-363, forms waveguide materials, from polyimide and polystyrene on a silicon dioxide substrate, which guide light from gallium arsenide LEDs and to silicon photodetectors.
Hartman, Howse, Krchnavek and Ladany, in an article entitled "Patterned channel waveguides on printed circuit boards for Photonic Interconnection Applications", Technical Digest, Topical Meeting on Integrated and Guided Wave Optics, paper no. MC4-1, pp 63-65 (1988), describe the fabrication of waveguides from "commercially available ultraviolet curing adhesives . . . The materials have a glass transition temperature of -10 degrees centigrade, but they maintain their adherence properties to at least 100 degrees centigrade." Later they state "There are a host of radiant curing optically clear adhesives available in the industry. Many have lower glass transition temperatures and higher temperature ranges of operation. We are currently evaluating these materials."
Hartman, Lalk, Howse and Krchnavek, in an article entitled "Radiant Cured Polymer Optical Waveguides on Printed Circuit Boards for Photonic Interconnection Use", Applied Optics, Vol. 28, No. 1, January 1989, pp. 40-47, describe the fabrication and evaluation of patterned channel waveguides formed on printed circuit card material by use of ultraviolet light cured adhesive films as channel waveguide material. They state:
"Because the waveguide materials were formulated as adhesives (and often called epoxies, albeit inaccurately), films can be adhered to many types of surface--clearly Teflon composites are one of the most difficult. As long as complete curing of the film is accomplished, through proper intensity-exposure time combinations and avoidance of unreasonably thick films (&gt;250 .mu.m), adhesion has not been a problem." PA1 placing on at least part of a surface of the substrate a primer precursor layer PA1 comprising an acrylic monomer containing at least two unsaturated groups per PA1 molecule of the monomer; and PA1 curing the primer precursor layer; PA1 (a) forming on at least part of a surface of a substrate a first layer comprising at least one ethylenically unsaturated monomer; PA1 (b) curing the monomer containing layer to form a first polymeric layer having a first T.sub.g (glass transition temperature); PA1 (c) heating the assembly to a temperature above the T.sub.g of the first layer for a period of at least 15 seconds; and PA1 (d) further curing the assembly. PA1 (1) carrying out on each or selected ones of the devices a test which consists of: PA1 (2) then determining a final optical attenuation (A.sub.2) of the device at the selected wavelength: PA1 (3) then determining whether A.sub.1 is greater than 3 dB per cm and whether A.sub.2 -A.sub.1 is greater than 1.5 dB per cm; and PA1 (4) rejecting those devices or batches containing those devices that have A.sub.1 greater than 3 dB per cm and/or A.sub.2 -A.sub.1 greater than 1.5 dB per cm. PA1 (1) heating the waveguide structure in an oven under vacuum at 150.degree. C. for 4 hours to remove volatile materials; PA1 (2) determining an initial optical attenuation (A.sub.1) of the waveguide structure at a selected wavelength; PA1 (3) then placing the waveguide structure in an oven heated to a temperature of 300.degree..+-.3.degree. C. for a period of 3 minutes; PA1 (4) then removing the waveguide structure and allowing it to cool to room temperature; PA1 (5) then determining a final optical attenuation (A.sub.2) of the waveguide structure at the selected wavelength,
Substrates were Teflon.TM. composite PC board material, epoxy-fiberglass board, aluminum ceramic, glass and silicon. Commercially available adhesive curable compositions such as Electrolite 4481 and Norland 63 were used to prepare the waveguides.
In the fabrication of acrylic or other polymer based channel waveguide and rib waveguide structures, it is advantageous and often essential to have extremely thin polymer films incorporated into the completed structure. There are various semiconductor type processing methods which can be used for creating thin films which can then be ultraviolet cured to make acrylic based waveguide structures. One important method which can be used is spin coating. Spin coating involves dispensing the material onto a wafer upon which the waveguide structure is being fabricated. The wafer is then rotated at a speed that will yield the desired film thickness and the coating then cured. Another important method of creating thin films is plasma etching. By way of example, an oxygen plasma (for organic materials), run under isotropic conditions, may be used to reduce a thick acrylic film evenly to a desired uniform thickness.
In connection with the formation of optical waveguide structures in polymer substrates, materials forming the channel waveguide media must satisfy a number of materials requirements and manifest certain requisite properties. Among the numerous properties such materials must possess are: high optical transparency at the wavelengths of interest (especially the 550-1550 nanometer spectral region), rapid and complete cure characteristics, workable fluid phase precursor consistencies prior to placement and cure, and selectable/controllable refractive indices. These materials must also be able to adhere securely to various substrates such as polyimide, gallium arsenide, indium phosphide, silicon nitride and crystalline silicon, which, in general, are difficult to adhere to. They must also show good adhesion between layers, i.e., good interlayer adhesion. Optical waveguide structures made from such materials must not show significant signal loss (attenuation) after being subjected to thermal cycling.
Ultraviolet light curable acrylic polymers have been proposed in the prior art, by Hartmann and coworkers, as indicated above, for channel waveguide structures. Thalacker and Boettcher in "Radiation Curing for Thermal Stability", Radiation Curing, November 1985, pp. 2-8 report that the thermal stability of acrylic resins can be improved by using polyfunctional acrylates.
Heretofore, the adhesion issue has been a significant problem. The various substrates noted above are particularly difficult to adhere to. Also, adhesion between layers of ultraviolet light cured resins is generally recognized to be a major problem. Moreover, the use of polyfunctional acrylates to improve the thermal stability of the acrylic resin exacerbates the adhesion problem for one result of crosslinking acrylic polymers is to reduce their ability to adhere and to be adhered to. Thus a hitherto unsolved need exists for fabricating methods and materials which manifest the requisite physical and optical properties while also manifesting greater adhesion to substrates of the above-indicated types.
In certain applications which are described in copending, commonly assigned, concurrently filed U.S. patent application Ser. No. 07/686,230, filed Apr. 15, 1991 and especially when the channel waveguide structures are to be secured to the difficult substrates referred to above, the waveguide materials are required to withstand temperatures of 300.degree. C. or higher for short periods of time. We have found that this requirement precludes the use of most if not all of the abovementioned known polymer candidate optical waveguide materials.
While the foregoing comments establish the existence of a high interest and activity level surrounding optical waveguide technology, the prior approaches have not been completely satisfactory, and the present invention provides significant and unexpected improvements applicable to this technology in order to satisfy the materials and process requirements noted above.